The present invention relates to an image recording printing plate precursor, which can be used as an offset printing master. More specifically, the present invention relates to a positive working photosensitive lithographic printing plate precursor for infrared laser suitable for so-called direct plate-making, which is capable of producing a printing plate directly by recording digital image signals formed by and output from a computer with an infrared laser beam.
As systems conducting plate-making directly based on digital data of computer, there have been hitherto known (1) system using an electrophotographic method, (2) photopolymerization system using a combination of exposure with Ar laser and post-heating, (3) system using a laminate composed of a silver salt light-sensitive material on a photosensitive resin, (4) system using a silver salt diffusion transfer material, (5) system using destruction of a silicone rubber layer with discharge or a laser beam, and the like.
However, these systems are accompanied with some difficulties. Specifically, the system using an electrophotographic method of (1) has problems in that the process including electrostatic charge, exposure, development, etc. is complicated and in that a complex and large-scaled machine is employed. The system of (2) requires not only the post-heating step but also a highly sensitive printing plate material, which causes difficulty in handling in a light room. The systems of (3) and (4) have problems of complicate processing and high cost due to employing the silver salt. The system of (5) has a problem in removing silicone residue remaining on the surface of printing plate material, although it has a relatively high percentage of completion. On the other hand, with remarkable progress of laser technologies in recent years, high output and compact solid laser or semiconductor laser devices having a light emission region in the range from near infrared to infrared become easily available. These laser devices are very useful as a light source for exposure in the production of a printing plate directly from digital data of computer or the like.
In a conventionally known positive working lithographic printing plate precursor material for infrared laser suitable for direct plate-making, an aqueous alkali solution-soluble resin having phenolic hydroxy group, for example, a novolak resin, is used as an aqueous alkali solution-soluble polymer compound. In such a positive working lithographic printing plate precursor, the association state of cresol novolak resin is changed by the action of heat generated from a light-heat converting agent in the exposed area to cause discrimination in solubility between the exposed area and the unexposed area, thereby carrying out plate-making. However, since the difference in dissolution speed between the exposed area and the unexposed area is small, a slight fluctuation of the alkali activity of a developer is apt to adversely affect the actual plate-making operation, resulting in lack of stability.
Thus, development of techniques for enlarging development latitude using an alkali dissolution interceptor (dissolution inhibitor) has been actively made. For instance, a resist material containing as the dissolution inhibitor, a (phenol pendant) acetal polymer having a specific structure is disclosed in WO 01/09682A2, a composition containing as the dissolution inhibitor, a polymer having a specific pendant group (including a hydroxy group or a carboxylic acid group) is proposed in WO 99/11458A, a printing plate precursor containing as the dissolution inhibitor, a polymer having at least one of ortho carboxylic acid, acetal and ketal graft chain and a photo-acid generator is disclosed in JP-A-10-87733 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), and an image recording material containing as the dissolution inhibitor, a hydroxyacryl polymer partially esterified is disclosed in European Patent 1,072,404A. However, these printing plate precursors are still insufficient in the discrimination.
An image recording material comprising an aqueous alkali solution-soluble resin having phenolic hydroxy group, for example, a novolak resin, a substance that absorbs light to generate heat, various onium salts and quinonediazide compounds is proposed in JP-A-7-285275. In such an image recording material, while the onium salt or quinonediazide compound functions as the dissolution inhibitor for substantially decreasing the solubility of the aqueous alkali solution-soluble resin in the exposed area, it is decomposed by heat in the exposed area so that it cannot act as the dissolution inhibitor, whereby an image is formed. However, since the onium salt or quinonediazide compound has light absorption region (350 to 500 nm) in a visible range, a place for handling the image recording material is disadvantageously restricted to a place illuminated with a yellow light. Also, a printing plate obtained is required to reuse by cleaning with a solvent, for example, a cleaner solution in order to use the printing plate for a long period of time. However, since the novolak resin is inferior in the solvent resistance, the printing plate has a problem in that printing durability is degraded when the cleaner solution is used during printing.
It is also described in JP-A-7-285275 that an acrylic resin or urethane resin can be used together with the onium salt or quinonediazide compound. Since there is no description regarding dissolution inhibiting or dissolution accelerating property of the acrylic resin or urethane resin to the aqueous alkali solution-soluble resin having phenolic hydroxy group in the patent, the addition of acrylic resin or urethane resin is not intended to dissolution inhibition of the aqueous alkali solution-soluble resin having phenolic hydroxy group. It is disclosed, however, that durability of the printing plate can be improved by the addition of acrylic resin or urethane resin.
It is described in U.S. Pat. No. 6,152,036 that insolubilization of surface layer due to a hardening reaction of novolak is also effective for the enlargement of development latitude and the hardening reaction is accelerated by adding an epoxy compound. Further, in JP-A-11-338131, it is disclosed that the development stability is improved by using an anionic metal complex having a heat-decomposable counter cation, as an infrared absorbing agent.
In JP-A-10-153863, it is disclosed that the stability of development is improved by using a specific metal complex or salt together with a specific binder having a hydrophilic group. It is disclosed in JP-A-2001-92126 that an organic compound containing an inorganic salt of metal selected from metals of Group IIIA of the periodic table and transition metals is used as the dissolution inhibitor, although the mechanism thereof is not clear. It is believed that the increase in discrimination of solubility described in these techniques disclosed is caused by the generation of dissolution inhibiting effect due to interaction between the binder and the metal compound. On the other hand, it is recognized to decrease the development stability and reproducibility of development latitude by means of influence to stability of the coating solution and influence of thermal history at the drying after coating to performances, which may also result from the interaction between the binder and the metal compound.
As described above, in the conventionally known positive working lithographic printing plate precursors for infrared laser suitable for direct plate-making, the approach for increasing the discrimination in the solubility between the image area and the non-image area fails to successively combine with practically necessary characteristics, for example, solvent resistance, safety to visible light, durability and production stability.
The present invention has been made against such a background. Therefore, an object of the present invention is to improve image-forming property, production stability and plate-making workability of a printing plate precursor containing an aqueous alkali solution-soluble resin having phenolic hydroxy group and a light-heat converting agent thereby resolving the above-described problems of conventional techniques. Specifically, an object of the present invention is to provide a positive working photosensitive lithographic printing plate precursor for infrared laser suitable for direct plate-making, which is excellent in stability of the coating solution, has no restrictions on the place of handling, and exhibits good discriminative developability, development stability, development latitude and press life.
Other objects of the present invention will become apparent from the following description.
As a result of extensive investigations to resolve the above-described problems, which damage practical characteristics of lithographic printing plate precursors, it has been found that incorporation of a polyvinyl acetal resin including an acid group having a pKa of not more than 5 into an image-recording layer of a lithographic printing plate precursor is effective to remove the above-described disadvantages. The present invention has been accomplished by further investigations based on such a finding. More specifically, the present invention provides the lithographic printing plate precursors described below.
1. A lithographic printing plate precursor comprising a support having provided thereon an image-recording layer containing a substance that absorbs light to generate heat, an aqueous alkali solution-soluble resin having phenolic hydroxy group and a polyvinyl acetal resin including an acid group having a pKa of not more than 5.
2. The lithographic printing plate precursor as described in item 1 above, wherein the polyvinyl acetal resin is a polyvinyl acetal resin having an ester linkage group with an organic acid selected from the group consisting of 1,2-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, phthalic acid and trimellitic acid.